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JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, Vol. 12, No. 5, May 2010, p. 1189 - 1193
Structural and electrochemical characterization of
TiO2/Pt hybrid catalyst system for direct bio-ethanol
fuel cell
A. BANU, N. SPATARUa, V. S. TEODORESCUb*, A. V. MARALOIUb, I. VOICULESCU,
A. M. MARCUa, T. SPATARUa
University Polytehnica Bucharest, Faculty IMST, Splaiul Independenţ ei 313, Bucharest, RomaniaaInstitute of Physical Chemistry “ I.G.Murgulescu” Romanian Academy, 202, Splaiul Independenţ ei, 060021, Bucharest 6,
RomaniabNational Institute of Materials Physics, Atomiştilor 105bis, 077125 Bucharest-M ă gurele, Romania
This paper is focused on preparation and microstructure and electrochemical characterization of carbon-TiO2/Pt catalystused as electrode in fuel cells applications. The platinum deposition was comparatively studied on graphitized carbonsubstrate and carbon substrate covered with titanium oxide. A strong influence on the platinum deposition, depending of theTiO2 presence, was found. The platinum nanoparticles about 5 nm in size are dispersed in aggregates of 50 to 100 nm on
the catalyst surface. This dispersion is strongly affected by the titanium oxide and the substrate porosity. The Pt/TiO2/G andPt/G electrodes were tested for their catalytic activity in the oxidation of ethanol. The prepared Pt/TiO 2/G electrode showhigher active surface area than for the Pt/G electrode, this increase in the surface area could be explained to be due to aspreading of wetting or the formation of new active sites at the Pt/oxide interface.
(Received April 15, 2010; accepted May 26, 2010)
Keywords: TiO2/Pt catalyst, Electron microscopy, Electrochemical characterization
1. Introduction
Pt and Pt-based catalysts have been extensively
investigated as electrocatalysts for the oxidation of liquid
fuels such as methanol and ethanol. However, the high
price and limited supply of Pt constitute a major barrier to
the development of direct alcohol fuel cells - DAFC.
For the best performance, DAFC electrode should
consist of two or more phases, such as a nanosized catalyst
and a porous material as a support for the catalyst. The
general requirements of support materials for their
applications to platinum-based electrocatalyst are a high
surface area for a high level of dispersion of nanosized
catalysts, a pore structure suitable for maximum fuel
contact and by-product release, and good interactions
between the catalyst nanoparticles and the support [1,2].
In particular, oxide supports are widely used in
heterogeneous catalysis and havean excellent stability in oxidizing environments [3].
Recently, more works have been focused on the system of
combining metal oxides, carbon and Pt [4,5].
Until now, titanium oxides have been potentially
attractive as supports due to strong catalyst-support
interaction, stability in fuel cell operation atmosphere, low
cost, commercial availability and the ease control of size
and structure [6.7].
The present paper has as a main objective the
structural characterization of titanium oxide platinum
hybrid catalyst system electrochemically obtained on a
graphitized carbon surface.
2. Experimental
2.1 Catalysts preparation
Two types of catalyst were prepared: the first one,
electrodeposited platinum on graphitized carbon sheet
(Pt/G) and the second one, hybrid titanium oxide/platinum
on a similar supporting electrode (TiO2/Pt/G).
The preparation of the Pt/G catalyst was done from a
0.1M HClO4 and 2.4mM H2PtCl6 solution at a constant
potential of -0.1 V/AgCl on a working surface area of
0.7cm2
for 180s. After platinum deposition, the electrode’s
surface was rinsed with bi-distilled water and dried at
room temperature.
The preparation of the second catalyst Pt/TiO2/G wasalso done by electrochemical method in two steps. In the
first step, the electro-deposition of TiO at a constant
potential of 0.7 V/AgCl on carbon substrate from 10 %
TiCl3 and 0.1M KCl 0.1M solution at the pH of 2.2 for
200s was performed. The pH value correction was realized
using saturated Na2CO3 pro analysis grade. After TiO2
deposition, the electrode was rinsed with bi-distilled water
and covered with platinum using electrochemical method
described above.
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1190 A. Banu, N. Spataru, V. S. Teodorescu, A.V.Maraloiu, I. Voiculescu, A. M. Marcu, T. Spataru
2.2 Electrode characterization
The electrodeposited Pt and TiO2/Pt electrodes were
characterized by surface structural analysis and
electrochemical techniques.
The surface structural analysis of the electrodeposits
was achieved by scanning electron microscopy (SEM)energy dispersing X-ray spectrometry (EDX), transmission
electron microscopy (TEM), and selected area electron
diffraction (SAED). The TEM specimens were prepared
by extracting micrometric fragments from the sample
surface and mounting these fragments on holey carbon
grids for TEM.
Jeol 200CX and INSPECT S FEI electron
microscopes were used for investigations and the images
processing was performed with AnalySIS Olympus
software for phase analysis.
In addition, the catalytic responses toward ethanol
oxidation were measured by chrono-amperometry
technique in a 3.75 M ethanol in 0.5M H2SO4 solution of.
3. Results and discussion
3.1 Microscopic characterization
3.1.1 Pt/G electrodeposition
The graphitized carbon has a major advantage as a
supporting electrode because it’s homogeneous structure at
the micrometric scale. However, at the nanometric scale
this material presents an interesting morphology consisting
in aggregates of nanocrystalites of graphitized carbon of
about 100-300 nm in diameter as shown in figure 1,
linked by quasi-amorphous carbon, where the graphiticplanes are in a random arrangement.
Fig. 1. TEM image and the corresponding SAEDshowing the nanostructure of the graphitized carbon
supporting electrode.
Fig. 2. The SEM image of carbon electrode decorated with small platinum particles
After platinum electrodeposition from hexachloride
platinum acid (H2PtCl6) solution at -0.1V/AgCl, the
electrode surface shows a relative homogeneous aspect
with small platinum particles (Fig. 2).
Using image analysis software AnalySIS Olympus,
was appreciate the coverage percentage of the carbon
electrode surface with platinum at about 10-11%. That will
not influence significantly the real area of the surface and
its roughness. The platinum particles are present on the
catalyst surface in quite large aggregates, about 50 – 100
nm in size as show in figure 3. However, the average size
of the platinum nanoparticles are about 5 nm, as can be
seen in high resolution image in figure 4 showing some
platinum particles dispersed in the quasi-amorphous
carbon matrix.
Fig. 3. TEM image and SAED pattern of platinumaggregate on carbon substrate.
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Structural and electrochemical characterization of TiO2/Pt hybrid catalyst system for direct bio-ethanol fuel cell 1191
Fig. 4. High resolution TEM aspect of platinumnanoparticle enbeded in the graphitized carbon matrix,which show the (0001) lattice fringes (0.335nm)
characteristic for graphite.
3.2 Hybrid TiO2/Pt Electrodeposition
As is shown in the figure 5, the graphitized carbon
electrode surface is quasi totally covered with TiO2. The
microanalysis of this coating shows a very small area of
micropores, which was appreciated by image analysis at
about 5% from the electrode surface.
Fig. 5 Low magnification SEM image of the electrodecovered surface with TiO2 coating by electrochemical oxidation from TiCl 3 based solution at 0,7V/AgCl
Titanium oxide and carbon are hydrophobic materials.
Because its well wet-ability on carbon substrate, the initial
TiO2 oxide thin film tend to cover the surface. However,
the electrodeposited titanium oxide coating consists from
nanosized hydrated TiO2 particles [8], can influence its
hydrophility. The high magnification SEM analysis of the
surface show a non homogeneous grows and a porous
structure formation in the coating out layer (see figure 6).
The size of the deposited TiO2 particle aggregate grows
three dimensionally from hundred of nanometers to
micrometer, forming a porous surface morphology.
Fig. 6. SEM image of the three- dimensional grow of titanium dioxide coating.
The electrochemical deposition of platinum
catalyst on TiO2 covered electrode is in this case available
only on non covered carbon sites areas as is shown in
Fig. 7.
Fig. 7. SEM image of platinum electrodeposited catalyst in a micrometric porous sites of the carbon electrode
with TiO2 coating.
The phase’s analysis using Olympus software reveals
very small platinum coverage degree of about 3%. That is
in a well concordance with carbon surface area non
covered with titanium oxide, estimated at about 5% ,
mentioned above. The EDX analysis of platinum covered
sites show a very small TiO2 signal compared with
platinum and carbon signals, as shown in figure 8.
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1192 A. Banu, N. Spataru, V. S. Teodorescu, A.V.Maraloiu, I. Voiculescu, A. M. Marcu, T. Spataru
Fig. 8. EDX Spectrum of TiO2/Pt hybrid system on theplatinum covered sites.
Because of the very small electrochemical working
surface exposed by the TiO2 non covered porous sites, we
suppose a big current density on these sites and a biggerplatinum particle aggregates formation, up to 500 nm in
size , as seen in Fig. 7.
Fig. 9. High magnification TEM image showing platinumparticle aggregate on the TiO2 particles.
However, the TEM study of some fragments extracted
from the Pt/TiO2/G electrode, show the presence of small
aggregate of platinum on the TiO2 nanoparticles ( figure
9). This means that some platinum particles are deposited
or dispersed in smaller pores of the covered electrode, non
visible on the SEM low magnification images.The TEM study revealed also that the titanium oxide
deposition is composed from crystallites of anatase and
rutil having sizes between 30 and 80 nm.
3.3 Electrochemical characterization
From the CVs (fig 10), can be seen that the platinum
oxide potential for the Pt/TiO2/G electrode increases
compared to those of Pt/G, which is in agreement with the
literature data [8]. The real electrochemical active surface
area, Ar, and Roughness factor, Rf calculated from the
region of hydrogen desorption [9] are summarized in table
1 [**]. The Pt/TiO2/G electrode shows higher active
surface are than Pt/G, probably because of wetting
spreading [10] or because of new active sites at the
Pt/oxide interface formation [11].
Table 1. Roughness factor of Pt/G and TiO2/Pt/Gelectrodes as determined from charges corresponding to
the hydrogen adsorption peaks [**].
Electrode Ar
( cm2/mg)
Rf
(cm2Pt/cm
2)
Pt/G 141,87 11.109
Pt/TiO2/G 323,56 34.557
.
Fig. 10. CVs of the graphite (1), Pt/G (2) and thePt/TiO2/G electrodes (3).
The real surface area of a catalyst can be orders of
magnitude greater than the geometric area. Since the
extent of adsorption and catalytic reaction rates depend on
real surface area, it is important to measure this value. A
measure of the real electrochemical surface area can be
obtained from the charge corresponding to the area under
the hydrogen adsorption peaks.
3.4 Ethanol electrooxidation
The as-prepared Pt/TiO2/G and Pt/G electrodes were
tested for their catalytic activity in the oxidation of
ethanol. Figures 11 and 12 shows cyclic voltammetry
curves recorded for both electrodes in a 0.5 M H2SO4 solution containing 3.75 M C2H5OH.
It can be seen that Pt/TiO2/G displayed a 2.8 times
higher current compared to Pt/G electrode. Considering
that Pt/TiO2/G and Pt/G had near Pt loading (see table1),
the Pt/TiO2/G appeared significantly more active for
ethanol oxidation. Furthermore, the onset potentials of
Pt/TiO2/G (0.35V vs Ag/AgCl) were lower than those of
Pt/G (0,2V) though the peak potentials were a little higher
( 0,55 V) than Pt/G ( 0,75V ), this fact indicated that
Pt/TiO2/G have higher catalytic activity for ethanol
oxidation at low potential than Pt/G.
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Structural and electrochemical characterization of TiO2/Pt hybrid catalyst system for direct bio-ethanol fuel cell 1193
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-50
-25
0
25
50
75
100
125
150
175
200
225
2
1
Current density/ A*g
-1 Pt
Potential / V
Fig. 11. Cyclic voltammograms for ethanol electro-oxidation at Pt/TiO2/G, (1) and Pt/G,(2) electrode in3,75 M C 2H 5OH and 0.5 M H 2SO4 solution with a scan
rate of 50 mV/s.
The peak current measured ethanol oxidation during
the forward scan, and corrected for the double layer
charging contribution, is generally used for the evaluation
of the so called “mass activity”, an important parameter
for the characterization of an electrocatalyst [13]. In our
case, for Pt/TiO2/g and Pt/G electrodes the mass activity
calculated from fig.3 was ca. 63.8 A/g , respective 15,9
A/g.
A plot of oxidation current vs. time
(chronoamperometry) was measured in 2.75 M ethanol +
0.5 M H2SO4 for the two electrodes at a constant potential
of 0.6 V vs. Ag/AgCl. As shown in Fig.12 the TiO2/Pt/G
electrode reveals much higher steady- state current than
Pt/G electrode. This is a direct evidence that TiO2/Pt/G
have higher catalytic activity then Pt/G electrode.
-10 00 0 1 00 0 2 000 30 00 4 00 0 50 00 6 00 0 7 00 0 8 000
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
i (A)
time (s)
2
1
1 Pt/G
2 Pt/TiO2/G
Fig. 12. Chronoamperometric response of Pt/G (1) and TiO2/Pt/G ( 2) electrodes of the ethanol oxidation in0.5 M H 2SO4 solution at an applied potential of 0.6 V vs.
Ag/AgCl.
4. Conclusions
The platinum nanoparticles, about 5 nm in size, aredeposited in the both electrodes, graphitized carbon andgraphitized carbon covered with TiO2, in aggregates withdimensions from 30 to 500nm. The electrodeposited
titanium oxide coating consists from crystallites of anataseand also rutil structure, with size between 30 to 80 nm.
The TiO2 electrodeposited layer shows a threedimensional growth forming a porous surface morphology.The phase’s analysis reveals very small platinum coveragedegree, about 3% for the TiO2/G electrode, in a well
concordance with carbon non covered surface by titaniumoxide, about 5%.Electrodeposited platinum catalyst on titanium oxide
porous coating is formed from very small nanosizedparticles spherical aggregates, comparing with the muchlarger platinum aggregates deposited in the TiO2 noncovered pores sites of the carbon electrode. As aconclusion of this work, the particles of platinumembedded in the film of titanium oxides have a bettercatalytic effect then the particles of platinum depositeddirectly on the support of graphite.
Acnowledgement
This work was supported from PN2 research contract
nr 21058/2007
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____________________________*Corresponding author: [email protected]